Get more from your slate investment.

Get more from your slate investment.

The problem

— Roofing slates trap water. Trapped water shortens slate life.

Trapped water

Between slates…

Slates trap water in familiar and unfamiliar ways. Most obviously, water is trapped between slates.

Because they overlap and touch, slates create capillary action, a force that drives water up between very close surfaces. While exposed slate surfaces dry in minutes after a rain, inspection reveals that water can linger for hours beneath slates. Slates rest, in effect, in their own puddles.

…within the slate…

Capillary action also works at the microscopic level. It can be powerful and intrusive. Fed by a persistent water source, capillary action can drive water from a tree's lowest root to its highest leaf. And, it can drive water through a stone's pores, commonly seen as rising damp in walls.

The implications for slate are significantt, particularly if the slate in question is characterized by high water absorption: Water trapped on one part of the slate can reach all of the slate.

…throughout the slate

Because of the geometry of overlapping slates, approximately 60% of the slate is subject to direct, persistent wetness. Because no part of the slate is more than the slate's thickness away from this wetness, virtually all of the slate is subject to water's effects.

No part of the slate is more than the slate's thickness away from persistent wetness.

Trapped water shortens slate life

Wet stones age; dry stones do not.

The archeological record shows that there is a proportional (and inverse) relationship between the time that building stone of any kind remains wet and its life span. Water itself is not typically injurious to slate, but the chemical and physical processes that it enables are.

Without water, calcite does not become gypsum, which swells and strains the slate's substrate; pyrite (often a misnomer) does not oxidize, which swells and then leaves voids; freeze-thaw cycles do not cause material fatigue, and acids do not attack.

Learn more about aging processes.


Different slates age differently

There are many types of slate, and water acts on them differently.

How a slate ages — how it reveals water's effects — reflects its mineral constituents, physical structure and propensity to absorb water.

Slates which possess little calcite and "pyrite", and which absorb little water, can well-resist normal exposure to water. On such slates, aging is largely confined to surfaces that most obviously trap water — surfaces that overlap and touch (below left). The structure of other slates makes them prone to aging at their edges (below middle). Still other slates, with higher levels of calcite, for example, can age pervasively (below right). In all cases, accelerating the rate of drying interrupts aging processes and lengthens slate life. (See "…within the slate…/…throughout the slate" above for a discussion of water transport within the slate.)

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